Development of secondary cells for use in portable electronic devices, hybrid vehicles, electric vehicles, or the like is conducted, and lithium ion secondary cells in particular are widely known as the most excellent secondary cell which operates at around room temperature. In such circumstances, lithium-containing olivine type metal phosphates such as Li(Fe, Mn)PO4 and Li2(Fe, Mn)SiO4 are not greatly affected by resource restriction and exhibit higher safety when compared with lithium transition metal oxides such as LiCoO2, and therefore become optimal positive electrode materials for obtaining high-output and large-capacity lithium ion secondary cells. These compounds, however, have a characteristic that it is difficult to enhance electrical conductivity sufficiently due to their crystal structures, and moreover, there is room for improvement in diffusibility of lithium ions, so that various kinds of development have been conducted conventionally.
Further, in lithium ion secondary cells the spread of which is progressing, a phenomenon is known that when the cells are left to stand for long hours after charge, the internal resistance gradually increases to cause deterioration in cell performance. The phenomenon occurs because water contained in cell materials at the time of production is desorbed from the materials during repetition of charge and discharge of the cell, and hydrogen fluoride is produced through the chemical reaction between the desorbed water and nonaqueous electrolytic solution LiPF6 with which the cell is impregnated. To suppress the deterioration in cell performance effectively, it is also known that it is effective to reduce the water content in a positive electrode active substance for use in a secondary cell (see Patent Literature 1).
Under the circumstance, for example, Patent Literature 2 discloses a technique for reducing the water content to a predetermined value or less by conducting pulverization treatment or classification treatment under a dry atmosphere after pyrolysis treatment of a raw material mixture comprising a precursor of a carbonaceous substance. Further, Patent Literature 3 discloses a technique for obtaining a composite oxide in which an electrically conductive carbon material is precipitated on the surface of the composite oxide uniformly by conducting mechanochemical treatment after mixing a predetermined lithium phosphate compound, lithium silicate compound, or the like with an electrically conductive carbon material using a wet ball mill.
On the other hand, lithium is a rare and valuable substance, and therefore various studies on sodium ion secondary cells using sodium in place of lithium ion secondary cells have started.
For example, Patent Literature 4 discloses an active substance for a sodium secondary cell using malysite type NaMnPO4, Patent Literature 5 discloses a positive electrode active substance comprising a sodium transition metal phosphate having an olivine type structure, and both the literatures show that a high-performance sodium ion secondary cell is obtained.